The changes in regional circulation which occur during labor will be studied in subhuman primates. Emphasis will be placed on the distribution of cardiac output (measured by dye dilution and by radio-active nuclide-labeled microspheres) to uterus, placenta, and other major organs during normal labor and during myometrial contractions induced by the infusion of prostaglandins (PGE2, PGF2 alpha) and oxytocin. The mechanisms which regulate myometrial blood flow vis-a-vis placental blood flow will be studied. The limitations of the primate placenta for gaseous exchange will be tested by comparing maternal-fetal placental perfusion ratios at rest and during labor. Measured characteristics of myometrial contractions (intensity, frequency, basal tonus) will be related to uteroplacental blood flow and fetal acid-base parameters in a preparation with chronic indwelling catheters. Maternal and fetal blood and amniotic fluid samples will be obtained during labor and assayed for oxygen, carbon dioxide, pH, and prostaglandins. The redistribution of systemic blood flow in the pregnant rhesus monkey will be examined in response to maternal stress and the hypothesis will be tested that a reduction in uterine blood flow leads to an increase in uterine tonus.